Copper and iron containing scale removal from ferrous metal



United States Patent Patented Nov. 8, 1960 ice solution than is needed to remove the incrustations. In such cases, it is more economical of solution to direct a 2,959,555 stream of it against the incrustations instead of filling the vessel. 5 The temperature of the acid solution at which the desired copper and iron oxide removal action is obtained,

Robert C. Martin, Tulsa, Okla., and William T. Abel, Toledo, Ohio, assignors to The Dow Chemical Company, Midland, Mich a corporation of Delaware No Drawing. Filed May 23, 1951, Ser. No. 0,999 l 6 Claims. (Cl. 252-149 The invention relates to a method of removing from a ferrous metal surface iron oxide incrustations containing copper such as the coppercontaining iron oxide incrustations oftentimes found on the internal surfaces of steam generators operated in conjunction with condensing equipment having a condensing surface of a copper alloy.

Attempts to remove copper-containing iron oxide incrustations of the type indicated above which are found quite frequently nowadays, especially in high pressure steam generators, as by chemical means, are fraught with difliculty. The use of hydrochloric acid solution is ineffective, because, although the iron oxides encountered can be removed by the acid solution, the copper component of the incrustations either is not removed or if removed by the solution is merely redeposited from it as a plating upon the ferrous metal surface. This difficulty in the use of an acid solution to remove boiler incrustations containing copper is disclosed in US. Patent 2,567,835 which provides a method for overcoming the difliculty. According to the patent, the incrustations can be removed by subjecting them first to the action of hydrochloric acid solution containing a corrosion inhibitor o owe y a wa er rinse; secondly tothc action of an a mpg oniacal sgl ution containing a strong oxidizing agent followed by aware. rinse; third to the action of dilute hydrochloric acid; and fourth to the action of a hot dilute aqueous alkaline solution. Although the foregoing method accomplishes the desired objective of removing both the iron oxide and copper components of the incrustations it has the disadvantage of requiring a multiplicity of steps which are not only laborious but also excessively time-consuming.

It is the principal object of the present invention to provide a method of removing from ferrous metal surfaces copper-containing iron oxide incrustations rapidly with a minimum of labor.

Other objects and advantages will become apparent from the following description of the invention.

According to the present invention, a copper-containing iron oxide incrustation on a ferrous metal surface, e.g. the inside of a steam generator, condenser, heat exchanger and the like, is removed by subjecting the incrustation to the action of a suitable acid solut'on in which is dissolved one of the followin f urea: thiourea, 1,3- dimethyl thiourea, hylene thiour 1,3-diefiyl iIiiourea, and 1,3diisopropyl t iourea. the iron oxides and copper-conta'ning incrustations may be used, for example, hgrochloric acid jprefer ablyjn a concentration of 5 to 25 rcent y weight), s (preferably in a concentration of 5 ercen an phosphoric acid (preferably in a concentrationbf 10 to 25 rcent).

*Ee acidsolution containing the urea derivatfve is put into contact, in any suitable manner, with the incrustation to be removed. In treating the inside of a vessel, for example, such as a steam boiler, it may be more or less filled with the solution so as to cover the incrustatious. Very large vessels may hold much more of the without redeposition of the copper, is between about 75 F. and 160 F. Because of the temperature requirement, it may essary in some cases to supply heat to 10 the solution or the equipment to be cleaned or to both the solution and the equipment to ensure having the incrustations and the solution in contact with them at a temperature at which they will be reacted upon the solution and removed.

Tests of the action of the solution on coppercontainin iron oxide incrustations indicate that the urea derivativ in the acid solution forms a precipitate or a complex with the copper which revents it from bein rede sited from the acid solution on the ferrous metal which redeposition occurs in he absgnce of the urea derivatiye.

The amount of the urea derivative required depends upon the quantity of copper in the incrustations to be removed and which becomes dissolved in the acid solution in the incrustation removal operation. The amount of 2 the urea derivative, we have found, should be at least about 2.0 unds r ound of co er to be brought into solution from the incrustations by the acid. Greater amounts of the urea derivative may be used if desired,

e.g. u to 20 unds .per und of co er. The preferred 3 opera mg amount of the urea derivative is between about 2.2 and 6 pounds per pound of copper to be removed. It is preferable to employ at least 0.01 percent of the thiourea derivative in the solution.

Test data set forth in the table are illustrative of the ,action of various amounts of a derivative of urea (e.g.

thiourea) in preventing the deposition of coppe m ld steel exposed to the descaling actio containing dissolved iron oxide a these tests, pieces of mild steel 49 were immersed for 6 hours in 150 ml. of 5 percent hydrocids capable of dissolving w chloric acid containing 0.4 percent of pyridine at 150 F. in which Fe O 1.33 percent by weight, and cuprous chloride, 0.33 percent Cu by weight, were dissolved.

Table Observations Test Percent No. Thiourea Ratio 1 Appearance of Appearance of Steel Piece Acid Solution 0 0 Cu plated on steel. 0. 53 1.6 Slight Cu platlng. Heavy precipitate in solution. 0.6 1.8 do Do. 0.67 2.0 Very slight Cu Light precipitate plating. in solution. 0.73 2.2 No Cu plating. Some precipitate in solution. 0. 87 2. 6 ....do Trace precipitate in solution. 0.93 2. 8 Do. 1.86 5.6 Do. 2.0 6.0 No precipitate in solution. 2.06 6.2 ....-do Do. 11 2. 2 6.6 do Do.

1 Ratio of the amount or the thiourea to that of the copper in the s: id solution.

6 From the foregoing data, it is manifest that as the concentrationof the urea derivative exceeds about 0.6 percent by weight of the acid solution and the ratio of the amount of e urea derivative to that of the copper in the acid ution exceeds about 2.0 plating out of copper from the acidsolution onto the steel is substantially suppressed. Also it appears that the urea derivative, at the lower concentrations, forms a more or less i c cid d cuprous chloride 15 insoluble compound with the dissolved copper, the compound becoming a precipitate in the acid solution; and at higher concentrations the urea derivative retains the copper in solution in some form of urea-copper complex, since the copper-containing precipitate formed at lower concentrations of the urea derivative does not form at the higher concentrations of the urea derivative. In either case, that is -whether the copper is converted into a precipitate in the acid solution or completely solubilized as on being complexed with the urea derivative, complete removal of the incmstations including the copper is obtained. In some cases, it is advantageous to use enough of the urea derivative to completely solubilize the copper as when the incrustations are on the inside of vessels of complex shape where the removal of a precipitate may be somewhat difiicult. In most instances, it is suflicient to employ but a few pounds of the urea derivative for each pound of copper to be removed.

Other urea derivatives already mentioned may be similarly used as exemplified in the comparative tests including blanks tabulated in Table II. In these tests mild steel coupons 2.75" x 1" x 0.125" were immersed in 5 percent hydrochloric acid solutions containing 0.128 percent of cuprous chloride and 0.4 percent of pyridine for 6 hours at 150 F. The appearance of the coupons after the 6 hours immersion was observed for the effect of various thiourea derivatives added to the solution on controlling copper deposition on the coupons.

Table II Thlourea Derivative 1 Appearance ot Coupon heavy copper plating.

thiouri a no copper plating. l,3 dimethyl thiouro negli ible copper plating. ethy1erethionrw. Do. 1,3-dietl1yl thiourea Do. 1,3-diisoprcpyl thiourea... Do.

l,3dibutyl thiourea 1 copper plating. di henylthinurea Do. ithiobiuret Do.

I Concentration 1.68 rroles per mole of copper. 1 When 1,3-dlbutyl thiourea is used in combination with thiourea no copper plating occurs.

In similar tests with other concentrations of copper and other ratios of the amount of the urea derivative to the amount of copper in solution similar results were obtained as noted in the tabulation of tests in Table In carried out in similar manner to those of Table H.

Although the acid solution containin i tive may be us Wl ouifgEQQQlliflhi-fiQL e presenmcorrosiofinhi nor of the organic nitrogenbase type has been found desirable especially when operating await -8011 Common orgamc m ogen bases which may be used as inhibitors are the well known coal tar bases produced in the distillation of coal tar. If desired the bases themselves may be separated from the crude coal tar base mixture and used as individual substances. These compounds are the aromatic and heterocyclic nitrogen bases soluble in the acid solution. The concentration or amount of the organic nitrogen-base inhibitor to employ depends upon its effectiveness and the extent to which it is desired to suppress the corrosiveness of the acid and the temperature at which the acid solution is to be used. Generally the amount effective to inhibit or reduce the corrosiveness of the acid is in the range of 0.1 to 1 percent of the weight of the acid solution.

"The following are examples of corrosion inhibitors from coal tar: pyridine, alpha-picoline, beta-picoline, gamma-picoline, 2-n-amylpyridine,- 4-n-amylpyridine, 2- hexyl-pyridine, 2,4-lutidine, 2,6-lutidine, 2,4,6-collidine, quinoline, lepidine, quinaldine. Other compatible organics rosin amines (see US. Patent 2,510,063 and 2,510,284).

nitrogen-base inhibitors are rosin amines and hydrogenated The following example is illustrative of the practice of the invention as applied to cleaning a steam generator (high pressure steam boiler) having an internal volume of about 20,000 gallons. Prior to cleaning, examination of the interior of the generator revealed extensive in crustations of ferrosoferric oxide estimated to amount to about 2000 pounds. Occluded in the incrustations was an amount of copper and copper oxide estimated to be the equivalent of about pounds of metallic copper. To remove these copper-containing iron oxide incrustations, the generator was filled with a water solution of hydrochloric acid containing 5 percent of HCl, 2400 pounds of thiourea in solution, and 0.4 percent of pyridine as a corrosion inhibitor. After filling the generator with the foregoing solution, it was heated so as to bring the temperature of the generator and the acid solution in it to about 150 F. and this temperature was maintaincd for about 6 hours. During this time the iron oxide and copper incrustations were dissolved by and retained in the acid solution without any redeposition of the copper on the internal surfaces of the generator. The resulting spent acid solution was drained out of the generator which was then flushed with water. Examination revealed that the incrustations were removed leaving the interior surfaces clean and copper free.

The so-cleaned generator was then refilled with a dilute soda ash solution (about 3 percent Na CO which was then boiled for two hours in the generator to remove any trace of the acid solution. The generator was then 1 drained and flushed with water. I

As is evident from the examples given herein, the solution of metallic copper in the acid solution occurs in the presence of ferric iron derived from the iron oxide in the incrustations. This reaction may be expressed by the equation:

in which the metallic copper is converted into a cuprous compound. In the presence of the urea derivative used in accordance with the invention the copper so dissolved is precluded from redepositing upon a ferrous metal surface. While we have found that most copper containing incrustations to be removed by the acid solution contain an adequate amount of ferric iron to oxidize the copper a deficiency, if any, of ferric iron in the incrustation can be made up by adding a suitable oxidizing agent to the acid solution.

For example, there may be used iron oxide (Po o, or Fe O or ferric chloride as an oxidizing agent in amount sufficient to meet the stoichiometric requirements of the foregoing equation.

Among the advantages of the method are that a single solution sufiices to remove the incrustations. Since only one cleaning step is involved, the down time of the equipment to be cleaned is reduced to a This application is a .continuation-in-part of our copending application Serial No. 612,894, filed September 28, 1956, now abandoned.

We claim:

1. The method of removing copper and iron oxide containing incrustation from a ferrous metal surface without redeposition of copper thereon which consists in bringing into contact with such surface an aqueous solution of a urea derivative selected from the group consisting of thiourea, 1,3dimethyl thiourea, ethylene thiourea, 1,3- diethyl thiourea, and 1,3-diisopropyl thiourea, said solution containing an acid capable of dissolving the iron oxides and copper, the amount of said urea derivative being from 2 to 20 pounds per pound of copper in the incrustation, said aqueous solution being maintained in contact with the incrustation for a time suflicient to substantially solubilize the said incrustation.

2. The method according to claim 1 in which the aqueous solution also contains an amount of an organic nitrogen-base other than thiourea and the substituted thiourea compounds defined in claim 1 capable of inhibiting the corrosive action of the acid on ferrous metal.

3. The method of removing a copper and iron oxide containing incrustation from a ferrous metal surface without redeposition of copper thereon which consists in bringing into contact with such surfaces an aqueous solution consisting of water in which is dissolved from 5 to 25 percent by weight of hydrochloric acid, thiourea, and from 0.1 to 1 percent of an organic nitrogen base other than thiourea capable of inhibiting the corrosive action of the acid on ferrous metal, the amount of the thiourea being in excess of 0.01 percent by weight of the solution and between 2 and 20 times the weight of the copper to be removed, the volume of the aqueous solution being sufiicient to remove the incrustations.

4. A composition for removing copper containing iron oxide incrustations from a ferrous metal surface consisting of a solution in water of from 2 to 25 percent by weight of hydrochloric acid, a urea derivative selected from the group consisting of thiourea, 1,3-dimethyl 6 thiourea, ethylene thiourea, 1,3-diethyl thiourea, 1,3-di' isopropyl thiourea in the amount of at least 0.67 percent by weight, and from 0.1 to 1 percent of an organic nitrogen base soluble in the solution selected from the group 5 consisting of the aromatic and heterocyclic nitrogen bases capable of inhibiting the corrosive action of the acid on ferrous metal.

5. The method according to claim 3 in which there is 1 included in the aqueous solution an oxidizing agent ca- 10 pable of oxidizing the copper in the incrustation to the cuprous state.

6. The method according to claim 5 in which the oxidizing agent is ferric chloride.

References Cited in the tile of this patent UNITED STATES PATENTS 1,961,096 Cunninghamaflazulifln May 29, 1934 1,996,730 Thomas et away. .ti m Apr. 2, 1935 %,049517 Saukaitis L.2..= J L -4--- Aug. 4, 1936 2,071,966 Bolton 2= li Feb. 23, 1937 2,215,077 Beekhuis et attazzittzt Sept. 17, 1940 42,220,451 Hunt -Ailzi'ztl- Nov. 5, 1940 .2,31s,559 Percival 2534:.12L215--- May 4, 1943 2,474,526 Healy et al. 12 .t June 28, 1949 -2,485,529 Cardwell et al.].ifhzliti- Oct. 18, 1949 2,617,771 Rucker .'.E.. l;'i Nov. 11, 1952 2,61s,603 Schaetfer 4 .52.;25151- Nov. 13, 1952 ),628,l99 Lowenheim .ihziil... Feb. 10, 1953 2,769,690 Mahan et 111.422.1315.- Nov. 6, 1956 FOREIGN PATENTS 504,269 Belgium 5. 25. July 14, 1951 

3. THE METHOD OF REMOVING A COPPER AND IRON OXIDE CONTAINING INCRUSTATION FROM A FERROUS METAL SURFACE WITHOUT REDEPOSITION OF COPPER THEREON WHICH CONSISTS IN BRINGING INTO CONTACT WITH SUCH SURFACES AN AQUEOUS SOLUTION CONSISTING OF WATER IN WHICH IS DISSOLVED FROM 5 TO 25 PERCENT BY WEIGHT OF HYDROCHLORIC ACID, THIOUREA, AND FROM 0.1 TO 1 PERCENT OF AN ORGNIC NITROGEN BASE OTHER THAN THIOUREA CAPABLE OF INHIBITING THE CORROSIVE ACTION OF THE ACID ON FERROUS METAL, THE AMOUNT OF THE THIOUREA BEING IN EXCESS OF 0.01 PERCENT BY WEIGHT OF THE SOLUTION AND BETWEEN 2 AND 20 TIMES THE WEIGHT OF THE COPPER TO BE REMOVED, THE VOLUME OF TE AQUEOUS SOLUTION BEING SUFFICIENT TO REMOVE THE INCRUSTATIONS.
 4. A COMPOSITION FOR REMOVING COPPER CONTAINING IRON OXIDE INCRUSTATIONS FROM A FERROUS METAL SURFACE CONSISTING OF A SOLUTION IN WATER OF FROM 2 TO 25 PERCENT BY WEIGHT OF HYDROCHLORIC ACID, A UREA DERIVATIVE SELECTED FROM THE GROUP CONSISTING OF THIOUREA, 1,3-DIMETHYL THIOUREA, ETHYLENE THIOUREA, 1,3-DIETHYL THIOUREA, 1,3-DIISOPROPYL THIOUREA IN THE AMOUNT OF AT LEAST 0.67 PERCENT BY WEIGHT, AND FROM 0.1 TO 1 PERCENT OF AN ORGANIC NITROGEN BASE SOLUBLE IN THE SOLUTION SELECTED FROM THE GROUP CONSISTING OF THE AROMATIC AND HETEROCYCLIC NITROGEN BASES CAPABLE OF INHIBITING THE CORROSIVE ACTION OF THE ACID ON FERROUS METAL. 